This invention relates to the field of manufacturing and shaping articles by electroforming and machining procedures utilizing electrode reactions. More specifically, this invention is directed to a method of preparing articles by electroforming, and electrodischarge machining and a composition for use therein.
Electroforming is becoming increasingly popular as a method of producing hollow metal parts that have accurate contours and dimensions. This is done by providing an electrically conductive substrate or mandrel that has the contours, dimensions and surface finish desired for making the finished hollow article. The mandrel is then made the cathode of an electro-forming circuit and the desired amount of metal is plated onto it to form a metal wall of sufficient thickness to be self-supporting. The mandrel and the article must then be separated from one another, a requirement which often imposes design restrictions limiting the production of complex shapes having undercut portions and reentrant sections.
In electroforming the electrolyte solution in which the electrically conductive cathode of predetermined pattern is immersed contains dissolved salts of the metal to be deposited and the anode is a suspended slab of the metal to be deposited. A relatively high current flow per unit area of the cathode takes place for rapidly and accurately building up a thick metal wall around the cathode of sufficient thickness to provide a self-supporting wall for the resultant hollow article.
In electrodischarge machining, metal is removed accurately in a predetermined pattern from an article which may be of intricate configuration through the action of a high energy electric discharge on the region of the conductive work piece where the predetermined pattern of metal is to be removed. A thin gap is maintained between the electrode probe tool and the work piece which serves as the other electrode, both of which are submerged in a fluid which is a very poor conductor of electricity, for example, a light oil. The work piece requires a low impedance electrical connection thereto for accommodating the desired relatively high current flow, and when the voltage across the gap becomes sufficiently high it discharges through the gap in the form of a spark at frequent intervals. Each discharge removes material from both the tool and the work piece which serve as the circuit electrodes. Through various modifications of electrodischarge machining intricate shapes and contours can be cut. In all cases, however, the material which is to be machined must be a fairly good electrical conductor and must have a low impedance electrical connection thereto, regardless of size or intricacy of configuration.
Metallic powders added as filler to thermosetting plastic materials can produce electrically conductive materials. Thermosetting plastics have had glass spheres added thereto as a bulking agent and can be electrically conductive when the spheres possess electrically conductive surfaces.
In order for any material to serve as the mandrel in an electroforming operation, it must be electrically conductive. Presently, electroforming and electrodischarge machining (EDM) is carried out using low melting point temperature metal alloys containing bismuth, lead and/or antimony, for example such as is commercially available under the trade designation "Cerro-Alloy" from Cerro Corporation. Such metal alloys have a melting point range of approximately 90.degree. to 150.degree. C. Those alloys in the low part of this range may be separated from the electroformed metal part using boiling water. However, in many cases, particularly where complicated configurations are involved, traces of those alloys containing bismuth, lead and/or antimony have been known to remain in cavities in the formed part, so that articles for which it is desired to obtain close internal tolerances may not achieve the desired tolerances when using this method. In some cases, a particle or small chunk of the alloy may remain lodged in a narrow cavity in a part of intricate shape. Where such metal part is intended to be operated in a critical high-temperature environment, such as in the aircraft industry, a further problem can arise. The low melting temperature alloy which is present can become alloyed during high temperature operation with the metal part itself, thereby changing the local contour and/or chemistry of the part, possibly rendering it defective.
In EDM where the low melting temperature alloy may be used to provide a low impedance electrical connection to a work piece of intricate shape which forms one of the electrodes, toxic fumes of lead, bismuth and/or antimony may suddenly become released into the atmosphere if the spark discharging from the electrode probe tool punctures through the work piece and strikes this alloy. Moreover, such alloys are rather heavy (weighing generally five to six times as much as the silver-coated glass sphere containing wax compositions disclosed herein) and are expensive to use.
Thus, it can be seen that there have been both advantages and disadvantages to methods of forming, using electrically conductive work pieces. For example, when finely divided metal powders, such as silver, copper, or zinc, are added to a thermosetting plastic, such as epoxy in order to make it conductive, the conductive resin becomes costly due to the expenses of the powder plus resin, and becomes heavy because of the density of the metal added. The thermosetting conductive plastics present a most difficult cleaning problem and can only be used once, thereby further augmenting the expenses of use. In some cases, the thermosetting conductive plastics are laboriously removed from electroformed articles by a vapor solvent exposure process which consumes many hours for completion, causing very slow, expensive production. In electroforming, one either has to limit the shape of the article produced to that which will permit the mandrel to be removed if higher speed production is desired, or use the low melting point metal alloy containing bismuth, lead and/or antimony. Such alloys, although separable, cannot be completely removed from the electroformed article, so that parts with critically close tolerances are difficult to manufacture by this method or may become defective by inadvertent subsequent thermal alloying effects in higher temperature environments.
Nevertheless, the relatively high current flow electrode reaction electroforming and EDM processes described above are increasing in popularity because of the advantages over conventional machine-tool forming processes for certain applications, such as for intricate shaped hollow metal articles and for those having apertures or recesses of complex configurations. Electroforming is a particularly fast growing industry because it enables the fabrication of complex parts with the reduction of conventional machining requirements. Thus, the need for different and improved electroforming and electrodischarge machining processes remains. It would be especially advantagesous if these processes could be reduced in cost.